Foundation Supportworks, Inc. Case Studies: 28th Street Retaining Wall

A 410-foot-long concrete cantilever retaining wall, located at 28th Street in Southeast Grand Rapids, Michigan, experienced more than 10 inches of lateral movement in some areas. The retaining wall is 16 inches thick and ranges in height from about 7 to 11 feet. There was no drainage media behind the wall and the retained soils were thought to be poorly drained, which was the likely reason for the wall movement. Concrete and asphalt pavement at the top of wall had shifted laterally with the wall movement and there was concern that a commercial building located as close as 14 feet from the top of wall, would experience movement with continued rotation of the retaining wall system. The owner of the commercial structure wanted to stabilize the retaining wall to prevent further movement. There was no geotechnical information available for the additional shoring design.

Helical tiebacks were selected for the wall stabilization because of the ability to install with smaller equipment, in limited access work areas and have a method to verify capacity during installation. Given the concerns regarding drainage behind the wall, the retained soil would be removed prior to installation of the tiebacks, which would allow placement of drainage media behind the wall. This also provided an opportunity to backfill with free draining soils. Having the retained soil removed prior to installation of the helical tiebacks also allowed installation of the tieback lead sections from behind the wall without having to core large holes through the concrete wall face. The wall loads required for the tieback design were 4.4 kips per linear foot. The design called for one row of tiebacks on 5-foot spacings installed at a 15-degree batter angle. The resulting design working tension loads on the tiebacks were 23 kips. Given the lack of geotechnical information, a factor of safety of 2.5 was used for the tieback design. Based on the tieback loading, the HA150 solid square shaft with a 10”-12” helix plate configuration was selected. The tiebacks were installed to a minimum length of 24 feet and minimum termination torques of 5,800 ft-lb to achieve torque correlated ultimate capacities of 58 kips. Actual tieback lengths ranged from 24 to 40 feet long to meet the termination torque criteria. With the 5-foot spacings, 82 tieback locations were required along the wall. The tieback installation was performed over a period of 10 days. 

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